As an aside, for efficiency reasons, I would select the sur_refl_b01 band from modis1 image collection and map cloudfunction on it rather than selecting the band on an image by image basis inside the function itself. Something like this (not tested):

Thanks for your answer. It partially solves the fact of running into the error that I mentioned, but now it refers to image.select(...).rename is not a function. Also, it used to run with the given code if I apply it to 1 dataset, but started bugging when I applied the merge.
– Wouter NeisinghMay 8 '18 at 12:55